At the age of 3, Olivia was diagnosed with an ultra-rare type of Angelman syndrome, thought to affect one in 60,000 children.

The syndrome is characterised by severe intellectual disability, speech and movement impediments, and seizures. It can be genetic, but testing of family members showed that in Olivia's case, it was random.

"She doesn't have any words, just ‘mama', ‘dada' and ‘baba'. No-one can tell us how many more words she will have," Ms O'Brien said. "She will never have a proper conversation with us. She'll never say, ‘I love you Mum', or anything like that."

There are three Angelman children known to be living in Palmerston North, and one in Wellington.

The English paediatrician who discovered it, Harry Angelman, coined the term "happy puppet syndrome" to describe his patients' happy and expressive demeanours.

The label is now widely rejected in favour of the more affectionate term, "angels".

Now 5, Olivia loves the Wiggles, dancing and swimming, and she has been learning sign language with her first day at school only weeks away.

The rosy-cheeked little girl's face lights up with glee as she demonstrates some of her favourite signed words, including "banana", "cat" and "dog".

The bright side of caring for a daughter with Angelman syndrome is her effervescent nature, which Ms O'Brien calls "infectious". "She is constantly in a good mood and her tantrums last all of about three seconds.

"A bonus is she's so huggy and kissy. She loves people and is really in tune with their emotions. It's very interesting - if her friend is upset, she is upset."

But it is likely that Olivia will always struggle. Her lack of fear of heights and a love for water had landed the family in a few sticky situations, Ms O'Brien said. "Every house we have ever lived in has been flooded."

United States researchers have begun recruiting participants for clinical trials for a drug that could potentially treat the syndrome.

Angelman children usually enjoy a normal lifespan and Olivia's family hopes that she may even find work or hobbies later in life. However looking after Olivia would be a "life-long job", Ms O'Brien said.

The family is building a home suitable for Olivia, which will include a special space for her older sister, Brianna, 8, to have to herself.

Olivia is due to start at St James' Catholic Primary School soon where she will have a teacher aide.

The family has been thrilled at the support of the school, which has built a bathroom specially for Olivia, and is holding a fundraising day for her.